Many applications need to detect the position and motion of multiple objects in three dimensional space. For example, methods for tracking the movement of different parts of the human body can be used in computer-aided design, training systems, virtual environments, medical diagnosis and therapy, computer game control, mobile platform coordination, and robotic control. Three dimensional tracking of animals may be used for behavioral and gait studies or wildlife tracking, and three dimensional tracking of the motion of physical objects may be used in security, inventory control, asset tracking and instrumentation systems.
In the past, human body motion has been recorded using vision instrumented rooms, inertial measurement units (IMU), active magnetic trackers, or ultrasonic sensors. All of these prior techniques have shortcomings. Vision-based tracking systems require a carefully controlled environment to ensure line-of-sight visibility of visual markers that are often obtrusive and must contrast with background objects. Ultrasonic transponders are bulky, require a power source, and have limited line-of-sight constraints and their performance is greatly compromised by typical environmental interference. Inertial measurement units are bulky, costly, require a power source, and are limited by bandwidth and accuracy. Other techniques are based on active RF systems, such as Ultra Wide Band (UWB) and spread-spectrum time-of-flight, or received signal strength (RSSI) measurements. These techniques require a potentially bulky transponder that need an on-board battery. There thus remains a need for improved methods and apparatus that can provide unobtrusive, low-cost tracking at high data rates.
Radar has long been used to track a multitude of targets such as aircraft, weather balloons, and cars, and employs many different modes of operation, such as Doppler, angle-of-arrival, and time-of-flight measurement. Transponders located on the targets respond to interrogation signals by returning data containing an encoded signature that uniquely identifies each target being tracked. Transponders are carried by almost all airplanes to provide air traffic control systems with aircraft identification data. See, for example, “Radar Signal Processing,” by Robert J. Purdy et al., Lincoln Laboratory Journal, Vol. 12, No. 2 (2000).
Radio Frequency Identification (RFID) tracking systems have also been developed in which active or passive tags are attached to objects to be identified or tracked. Active RFID tags carry their own power source, typically a battery, and can operate at longer ranges than passive tags which draw their power from a field that emitted from a nearby wireless radiation source. U.S. Pat. No. 6,353,406 issued to Lanzi et al. on Mar. 5, 2002 describes a dual mode tracking system that employs both active and passive transponders, calculates the distance of a tagged object from each of several interrogating antennas by measuring round trip signal time, and then calculates a position estimate for the tagged target by triangulation. Unfortunately the transponders used in the Lanzi et al. system employ complex solid-state electronic circuitry to respond to and process the incoming signal, to produce encoded response signals that uniquely identify each transponder, and to retransmit the encoded signals to the reading stations. The passive tags used in the Lanzi et al system, while they eliminate the need to carry an onboard battery, require circuitry for generating an operating potential to power the signal processing circuitry from wireless power source that must be close to the passive tags. The resulting passive and active tags are expensive to manufacture, tend to be bulky, exhibit slow response times which limit the system's ability to track rapidly moving objects in real time, have limited range and, in the case of passive tags, require a wireless power station to supply operating potentials to the on-board solid state signal processing circuitry.